Which statement correctly identifies an energetically active transport process?

• A. Simple diffusion moves substances down their concentration gradient without energy.
• B. Facilitated diffusion uses energy to move substances down their gradient.
• C. Active transport uses energy to move substances against their gradient via pumps or co-transporters.
• D. Osmosis moves water across membranes without energy.

Answer: (C)

Energetically active transport moves substances against their concentration or electrochemical gradient by using cellular energy. This requires energy input, typically from ATP hydrolysis, to power a pump that changes shape and pushes ions or molecules uphill. Pumps like the Na+/K+ ATPase actively maintain ion gradients, while co-transporters use the energy stored in one substance moving down its gradient to pull another substance uphill (for example, a sodium-glucose symporter uses Na+ moving down its gradient to bring glucose against its gradient).

That’s why the correct statement is the one that describes active transport as using energy to move substances against their gradient via pumps or co-transporters.

In contrast, simple diffusion and facilitated diffusion both move substances down their gradients without direct energy use, with facilitated diffusion simply speeding up movement through a membrane protein. Osmosis is the diffusion of water across a membrane, also not energy-requiring.